1. Field of the Invention
The present invention relates to a method for detecting a position receiving a light beam, and more specifically to a method for detecting a spot of a position receiving a laser beam so as to measure various levels or heights and other distances.
2. Description of the Prior Art
A technique which takes advantage of the straight travelling nature of a light beam to make a precise measurement of lengths, distances, etc. is known.
For example, as shown in FIG. 6, a light beam receiving plate 12 is placed at a certain distance from a laser oscillator 11 to receive a laser beam 13 for the purpose of measuring a relative distance between the laser oscillator and the light beam receiving plate. A reference line is established so that a measurement can be made based on this reference line. Furthermore, as shown in FIG. 7, a laser beam is received by a light beam receiving plate 14 instead of the light beam receiving plate 12 to measure displacements of the position on X- and Y-axes, for example, for controlling the posture of an automatic excavator at the time of tunnel boring work.
Such a conventional light beam receiving plate is ordinarily provided with semiconductor diodes, i.e., light detecting elements called photodiodes, as sensors for converting the intensity of the received laser beam into electric signals.
FIG. 8 shows a light beam receiving plate, which is a so-called divided type, for example, to be used for making a one-dimensional measurement as in level measurement.
This divided type light beam receiving plate consists of five photodiodes arranged in a vertical row at intervals of the order of a millimeter. Each diode is provided thereon with a light beam receiving plate 12a, respectively. The light beam receiving plate 14, which is adapted to detect two-dimensional displacements, as shown in FIG. 7, is generally provided with light detecting elements arranged in the form of matrix with X- and Y-axes.
As stated above, a conventional light beam receiving plate requires, regardless of whether it is used for a one- or two-dimensional measurement, that an increased number of light detecting elements be used in measuring a larger span or displacement along the X- and Y-axes.
When an attempt is made to measure a position of a spot on a line ten meters long with an accuracy of 1 mm, it is necessary for 10,000 light detecting diodes to be incorporated into the plate.
This is also the case with two-dimensional measurements. As a replacement for the divided type light beam receiving plate there is also a non-divided type light beam receiving plate available, which detects the differences in electric resistance corresponding to the positions receiving a laser beam. This non-divided type has to be subjected to variations in the intensity of a laser beam due to the measuring circumstances and also in the luminosity of the spread light spot, which results in a troublesome calibration being required. Especially, the accompanying processing circuits become complicated, and, further, the analog processing circuits of the non-divided type do not allow high precision.
Still further, one light beam receiving plate of the non-divided type is as large as up to several centimeters. For a measurement of a long span, a large number of these non-divided type light beam receiving plates have to be combined, resulting in a high price of the plate.
In addition, for a two-dimensional measurement, there is a method wherein an image obtained by a charge coupled device (CCD), an image pick up tube, etc. is processed to detect a spot receiving a laser beam. An increase in the number of the picture elements for the purpose of increasing accuracy will also increase the processing time per picture element ranging from several tens of seconds to several minutes. Therefore, this method is unsatisfactory for high speed processing and is impractical.
In any event, as long as a conventional method is employed, any attempt to expand the scope of measurement, whether it is of one dimension or two dimensions, will increase the number of light detecting elements with the consequence that the light beam receiving plate becomes very expensive and that the processing circuits become complicated.